1. Field of the Invention
The present invention relates to storage area networks and, more specifically, to distribution of storage elements across a wide area network.
2. Description of the Related Art
Recently, the growth in demand for networked storage has exploded. Enterprise applications such as data warehousing, email, and remote backup create hundreds of terabytes of data per day. Storage Area Networks (SANs) and Network Attached Storage (NAS) have become the de-facto technologies for storing, organizing, and distributing the massive amount of data being generated by these applications.
Until recently, storage area networks (SANs) were typically limited to a single site or to operations over a short distance or over low-bandwidth bridges. As the demand for networked storage continues to grow, so does the demand for connecting these “localized islands” of storage over local-area networks (LANs), metropolitan-area networks (MANs), and wide-area networks (WANs).
Traditionally, the need to connect SANs over long distances was driven by applications such as archiving and disaster recovery. Additionally, new time-sensitive applications such as remote web mirroring for real-time transactions, data replication, and streaming services are increasing the demand for high-performance SAN extension solutions.
Connecting SANs over short distances is a relatively straightforward task. Fiber channel (FC), the dominant protocol for SAN connectivity, provides the ability to connect SANs at distances up to 10 km.
An in-development alternative to FC is iSCSI. iSCSI (internet small computer systems interface), a new IP-based storage protocol that will be used in Ethernet-based SANs, is essentially SCSI over transmission control protocol (TCP) over Internet protocol (IP).
The fiber channel family of standards (developed by the American National Standards Institute (ANSI)) defines a high-speed communications interface for the transfer of large amounts of data between a variety of hardware systems such as personal computers, workstations, mainframes, supercomputers, and storage devices that have FC interfaces. Use of FC is proliferating in client/server applications that demand high-bandwidth and low-latency I/O such as mass storage, medical and scientific imaging, multimedia communication, transaction processing, distributed computing, and distributed database processing. More information about fiber channel can be obtained from the National Committee on Information Technology Standards (NCITS) T11 standards organization that regulates FC standards, specifically “Fiber Channel—Physical and Signaling Interface, FC-PH,” draft proposed standard, ver. 4.3, ANSI, 1994 (herein “the FC-PH standard”), incorporated herein by reference in its entirety.
Regardless of the technology (FC or iSCSI), performance is affected by many factors such as the distance between the data centers, the transport protocols (e.g., synchronous optical network (SONET), asynchronous transfer mode (ATM), and IP) and the reliability of the transport medium.
Many techniques have been developed to transport FC data through existing data and telecommunications networks. Boxes and systems are beginning to emerge that provide basic extensions of the physical links of FC. These basic-extension systems “stretch” the wire but often don't address system-level issues of FC performance (e.g., end-to-end flow control). Existing extension gateways can tunnel FC data over other transport protocols but only between a few sites. Fiber channel provides an inherently reliable method of transporting data. SCSI (which may ride on FC) presumes that data is delivered in order and reliably. To carry FC outside a data center, an extension system should maintain the same levels of performance and reliability regardless of the distances involved. Efforts focusing on solutions to FC long distance transport are documented by IETF draft standards in the areas of FC-over-IP (FCIP), specifically Rajgopal, M., “Fibre Channel Over TCP/IP (FCIP)”, Internet Draft, IPS Working Group, IETF, version 12, Aug. 2002 (herein “the FCIP draft proposal”), incorporated herein by reference in its entirety.
By its nature, IP is unreliable. Mechanisms exist to signal congestion conditions in IP networks but they are generally associated with the dropping of packets. One such mechanism is the ICMP quench message discussed in Postel, J., “Internet Control Message Protocol (ICMP),” Network Working Group, RFC 792, IETF, Sep. '81 (herein “RFC 792”), incorporated herein by reference in its entirety. Layering transmission control protocol (TCP) over IP corrects for errors in packet ordering and packet loss but does so with the insertion of considerable delay and with reduced bandwidth associated with the retransmission and window control mechanisms used in TCP. These mechanisms and their impact on performance for large data transfers over long distances are discussed in more detail in Semke, M., Mahdava, and Ott, “The Macroscopic Behavior of the TCP Congestion Avoidance Algorithm,” Computer Communication Review, a publication of the ACM SIGCOMM, vol. 27, number 3, 1997 (herein “Semke '97”), incorporated herein by reference in its entirety.
As enterprise storage needs continue to grow, solutions are needed that can extend the reliable, high-performance characteristics of FC to SAN fabrics of thousands of kilometers, thereby unifying enterprise SAN islands into large geographically dispersed SAN systems.